Refined Spin Wave Model and Multi-magnon Bound States in $Li_{2}CuO_{2}$

Abstract: Here we report a study of the spin dynamics in the ferromagnetic chain compound $Li_{2}CuO_{2}$. Inelastic neutron scattering measurements allow for the spin Hamiltonian to be determined using a $J_{1}-J_{2}$ $XXZ$-Heisenberg spin chain model with weak interchain interactions. The primary exchange parameters determined from our data are qualitatively consistent with those of Lorenz $et$ $al.$ [Europhys. Lett. 88, 37002 (2009)], and our data allow for the resolution of additional interchain exchange interactions. We also observe the formation of two- and, potentially, three-magnon bound states. The two-magnon bound state exists only in the magnetically ordered phase of this material, consistent with stabilization by the weak, Ising-like exchange anisotropy of the nearest-neighbor intrachain interaction. In contrast, the potential three-magnon state persists in a finite temperature regime above $T_{N}$, indicating an unconventional character. Our results establish $Li_{2}CuO_{2}$ as an experimental platform for the study of exchange anisotropy-stabilized bound states in a ferromagnetic chain.